Impact of rate control on the capacity of an Iub link: single service case

Universal Mobile Telecommunications System (UMTS) networks are capable of serving packet-switched data applications at bit rates as high as 384 kbps. This paper studies the capacity and utilization of the downlink of the Iub interface, which lies between the radio network controller (RNC) and the base station (NodeB) in the UMTS network. 

The 3GPP standards define a Node B 'receive window' within which a frame should arrive for it to be processed and transmitted to the UE in time. If the frame arrives too late, it will be discarded. Such frame discard event results in some loss in voice/data quality. Via simulations, we evaluate the link capacity for web-browsing traffic at 64 kbps, 128 kbps and 384 kbps, with a frame discard probability target of 0.5%. Our results indicate that the Iub link utilization is very poor due to the highly bursty nature of data traffic. 

In order to alleviate this problem, we introduce a rate control (RC) scheme where the peak user data rate is temporarily lowered during times of high congestion. This lowering of data rate is done through appropriate selection of the transport block size within the transport format set. As a result of such rate control, the capacity of the Iub link improves.